An official in China holds a bird infected with H7N9 in 2013, the year the virus was first detected

In the US scientists are not allowed to carry out work that could cause a pathogen like H7N9 to take on a new function or mutation that isn't already seen in nature.

As a result Prof Kawaoka's team cannot perform the experiments they need to, to find out why the new strains of H7N9 are lethal and resistant.

He recently said this temporary ban on this kind of work makes it challenging for scientists.

He and other colleagues said: "Results from studies would almost certainly help in understanding the pandemic potential of influenza viruses and produce public health benefits, such as the prioritisation and development of pre-pandemic vaccines and antiviral drugs."

For now, Prof Kawaoka said it is important countries improve their "surveillance" of H7N9.

The findings are published in Cell Host & Microbe.

MORE ON BIRD FLU

FUMES FEAR

E-cigarettes increase your risk of flu and make virus MORE deadly, experts say

KILLER FLU

Dogs could give deadly flu to humans causing pandemic which could kill millions